In recent years, along with the spread of mobile electronic devices such as mobile phones and notebook-sized personal computers, development of a small and light nonaqueous electrolyte secondary battery having a high energy density has been earnestly desired. In addition, development of a high power secondary battery has been earnestly desired as a battery for electric automobiles typified by hybrid automobiles. The secondary battery which meets the above requirement is a lithium ion secondary battery. The lithium ion secondary battery is composed of a negative electrode, a positive electrode, an electrolyte and the like, and a material capable of desorbing and inserting lithium has been used as an active material for the negative electrode and the positive electrode.
Research and development of the lithium ion secondary batteries have been extensively carried out at present. Among them, the practical application of a lithium ion secondary battery in which a layer or spinel type lithium metal composite oxide is used as a positive electrode material has been progressed as a battery having a high energy density, since the battery gives a high voltage as high as 4 V.
As a positive electrode material for use in the lithium ion secondary battery, there have been hitherto proposed lithium composite oxides such as lithium cobalt composite oxide (LiCoO2) which can be relatively easily synthesized, lithium-nickel composite oxide (LiNiO2) in which nickel being less expensive than cobalt is used, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), and lithium manganese composite oxide (LiMn2O4) in which manganese is used.
In order to impart favorable performance (high cycle characteristic, low resistance and high power) to a positive electrode, it has been required for the positive electrode material to be composed of particles having a uniform and appropriate particle diameter. This is because the reaction area of the material with the electrolyte cannot be sufficiently ensured when a material having a large particle diameter and a small specific surface area is used, and because there arise defects such as lowering of battery capacity and increase of reaction resistance when a material having a broad particle size distribution is used. Incidentally, the reason why the battery capacity is lowered is that the voltage applied to the particles in the electrode becomes uneven, and thereby finer particles selectively deteriorate when charge and discharge are repeated.
Additionally, it is effective for increasing the output of a battery to shorten the transferring distance of lithium ions between a positive electrode and a negative electrode. Therefore, it has been desired to thin the positive electrode plate, and therefore, cathode active material particles having a smaller particle diameter are useful for this desire.
Accordingly, it is necessary to produce the above-mentioned lithium-nickel composite oxide particles having a small particle diameter and a uniform particle diameter.
The lithium-nickel composite oxide is usually prepared from a composite hydroxide. Therefore, in order to prepare a lithium-nickel composite oxide particle having a small particle diameter and a uniform particle diameter, it is necessary to use a composite hydroxide having a small particle diameter and a uniform particle diameter as a starting material. In other words, in order to produce a final product, that is, a lithium ion secondary battery having high performance by improving performance of a positive electrode material, it is necessary to use a composite hydroxide composed of particles having a small particle diameter and a narrow particle size distribution as a composite hydroxide which is employed as the source material of the lithium-nickel composite oxide for forming the positive electrode material.
Patent Document 1 discloses a lithium composite oxide in the form of particles having a particle size distribution in which the average particle diameter D50, which means a particle diameter of the particles having an accumulation frequency of 50%, is 3 to 15 μm, and in which the minimum particle diameter is not smaller than 0.5 μm, and the maximum particle diameter is not greater than 50 μm in the particle size distribution curve; and in the relationship between D10 which means a particle diameter of the particles having an accumulation frequency of 10% and D90 which means a particle diameter having an accumulation frequency of 90%, D10/D50 is from 0.60 to 0.90, and D10/D90 is from 0.30 to 0.70. In addition, this document discloses that a lithium ion nonaqueous electrolytic solution secondary battery having excellent output characteristics and a small lowering of cycle characteristics can be obtained by using this lithium composite oxide, since the lithium composite oxide has high repletion property, excellent charge and discharge capacity characteristics and high power characteristics, and is not deteriorated even under the conditions such as a large charging and discharging load.
In addition, various processes for producing composite hydroxides have been proposed (see for example, Patent Documents 2 and 3).
Patent Document 2 proposes a method for producing a cathode active material for nonaqueous electrolyte batteries, in which a precursor, an oxide or a hydroxide is obtained by a process comprising charging a reaction vessel with an aqueous solution containing at least two kinds of transition metal salts or at least two kinds of aqueous solutions each of which contains a different transition metal with each other, and an alkali solution at the same time, and carrying out coprecipitation under the existence of a reducing agent or while blowing an inert gas into the solution.
Patent Document 3 also discloses a method for producing a cathode active material for a lithium secondary battery. This document discloses that lithium-coprecipitated composite metal salt particles having an approximately spherical shape are prepared with a reaction vessel by continuously feeding an aqueous solution of a composite metal salt in which the concentration of the salt is controlled by dissolving a salt having an element which constitutes the above-mentioned active substance in water, a water-soluble complexing agent which forms a complex salt with a metal ion, and an aqueous solution of lithium hydroxide to a reaction vessel, respectively, to generate a composite metal complex salt; thereafter decomposing this complex salt with lithium hydroxide, to extract a lithium-coprecipitated composite metal salt; carrying out the generation and decomposition of the above-mentioned complex salt repeatedly while circulating in the reaction vessel; overflowing the lithium-coprecipitated composite metal salt to take out. This document also discloses that the cathode active material in which the composite metal salt obtained in this process is used as a source material has a high packing density, homogenous components and a nearly spherical shape.
However, the lithium composite oxide disclosed in Patent Document 1 includes very fine particles and coarse particles, since the minimum particle diameter is 0.5 μm or more, and the maximum particle diameter is not greater than 50 μm in contrast to the average particle diameter of 3 to 15 μm. Therefore, it cannot be said that the range of the particle size distribution as defined by the above-mentioned D10/D50 and D10/D90 is narrow in the particle diameter distribution. In other words, since it cannot be said that the lithium composite oxide of Patent Document 1 have particles having uniform particle diameters, it cannot be expected to improve the performance of the positive electrode material even though the lithium composite oxide is employed, and it is difficult to obtain a lithium ion nonaqueous electrolytic solution secondary battery having sufficient performance.
On the other hand, Patent Document 2 discloses a method for producing a composite oxide. However, since crystals generated are collected by classification, it is necessary to strictly control the condition for producing. Therefore, it is difficult to produce in an industrial scale. Moreover, according to this process, even though particles having a large particle diameter can be obtained, it is difficult to obtain particles having a small particle diameter.
Patent Document 3 also discloses a continuous crystallization method which comprises taking out a product by overflowing. According to the method, since the particle size distribution becomes a normal distribution, and is likely to be spread, it is difficult to obtain almost uniform particles having a small particle diameter.
As described above, a composite hydroxide which sufficiently improves the performance of a lithium-secondary battery has not yet been developed. Furthermore, although various processes for preparing a composite hydroxide have been also examined, there has not yet been developed a process which enables to prepare in an industrial scale a composite hydroxide which can sufficiently improve the performance of a lithium secondary battery at the present time. Therefore, it has been desired to develop a process which enables to prepare this composite hydroxide.